The permeability to oxygen of silicone polymer networks with controlled high-molecular-weight side chains was measured using electrochemical techniques. Networks with controlled amounts of pendent chains were prepared by hydrosilation reactions involving the addition of hydrosilanes from crosslinking molecules to the vinyl end-groups of mono- and difunctional prepolymer molecules. Values of the permeability coefficient P were determined by measuring the steady electric current in an experimental set-up in which several layers of moistened paper were placed between the network and the electrodes. The curve representing the dependence of the permeability coefficient on the molecular weight of the dangling chains, M, shows that, for values of M < 130 000, P is nearly a linear function of M; however, for M > 130 000, the permeability coefficient does not show a noticeable dependence on the molecular weight of the dangling chains. The diffusion coefficient increases with the molecular weight of the side chains, reaching a nearly asymptotic value for M approximate to 170 000.